1. Field of the Invention
The present invention relates to an image recording apparatus suitable for copying, printing, plotting, facsimile reproduction, and similar applications, and more particularly to a technique for improved recording efficiency, reduced power requirement and enhanced quality of an image reproduced.
2. Discussion of the Related Art
An example of a known image recording apparatus is disclosed in U.S. Pat. No. 3,689,935 to G. L. Pressman et al. This recording apparatus is designed to form an image on a recording medium, by modulating particle flows of a toner through a plurality of apertures formed through a particle flow modulator. The particle flows through the apertures are modulated by applying controlled electric potentials to respective electrodes provided on the particle flow modulator according to image signals. Described in detail, the particle flow modulator includes an insulating layer, a shielding electrode in the form of a continuous conductive layer formed on one of opposite surfaces of the insulating layer, and a segmented conductive layer formed on the other surface of the insulating layer. The segmented conductive layer consists of a plurality of control electrodes which are electrically insulated from each other. The particle flow modulator has at least one row of apertures formed through the insulating layer and the continuous and segmented conductive layers, such that the apertures correspond to the respective control electrodes. The apparatus also includes: voltage applying means for applying selected electric potentials between the shielding electrode and each of the control electrodes; toner supply means for providing a crowd of electrostatically charged toner particles so that flows of the charged toner particles through the individual apertures of the particle flow modulator are modulated by the applied electric potentials; and means for positioning the recording medium in the path of flow of the toner particles and for providing relative translation between the recording medium and the particle flow modulator.
U.S. Pat. No. 4,912,489 refers to U.S. patent applications Ser. Nos. 946937, 926129, 140266 and 926158, which disclose printers of the type in which the particle flow modulator has control electrodes on the side of the recording medium, and a shielding electrode on the side of the toner supply means.
The U.S. Pat. No. 4,912,489 discloses a particle flow modulator having the reversed arrangement. Namely, the modulator has a shielding electrode on the side of the recording medium, and control electrodes on the side of the toner supply means. This Patent teaches an advantage of this type of particle flow modulator, that the control electrodes are roughly four times more effective than in the prior art device of the type indicated above, in repelling the toner in the off state, namely, when image dots are not to be formed. Thus, the control voltage necessary to modulate the flows of the toner particles through the apertures is about one fourth that required in the prior art.
The particle flows through the apertures will cause respective image dots to be formed by the toner particles on the corresponding local spots on the recording medium, while the inhibition of the particle flows through the apertures results in leaving the corresponding local spots non-imaged by the toner. Thus, an image is formed by modulating the particle flows of the toner through the individual apertures of the particle flow modulator.
In the conventional image recording arrangement disclosed in the U.S. Pat. No. 4,912,489, a layer of toner is supplied by a toner conveyor which travels under the row of apertures of the particle flow modulator (printhead structure). When an image signal for a given aperture requires the formation of an image dot, an appropriate imaging potential is applied between the corresponding control electrode and the shield electrode of the particle flow modulator, so that a crowd of the toner particles is passed through the aperture in question. However, when the image signal does not require the formation of an image dot, the potential applied is changed to a non-imaging value for inhibiting the passage of the toner particle through the aperture. In this off state, the crowd of toner particles is moved away from that aperture. Consequently, the density of the toner crowd near the aperture in question is considerably lowered. This is undesirable when the imaging potential is subsequently applied to cause the passage of the toner particles through that aperture. Thus, the response of the toner flows through the apertures to a change in the potential applied to the particle flow modulator is not satisfactory due to the movement of the toner particles away from the apertures when the non-imaging potential is applied.
Further, the known image recording apparatus indicated above more or less suffers from plugging of the apertures with the toner particles which are deposited on the surfaces of the control electrodes due to the effect of the image force. This leads to deteriorated quality of the image reproduced, that is, local failure to form image dots due to the plugged apertures.
The known image recording apparatus has another drawback, which arises from the arrangement for application of an electric potential to control the flows of the charged toner particles. That is, the potential is applied so that an electric field is produced within the aperture. Accordingly, the magnitude of the electric field outside the aperture is considerably small. This means a relatively small force for introducing the toner particles from under the aperture into the interior of the aperture, whereby the amount of toner which passes through the aperture per unit time is accordingly small, leading to a relatively long time required to allow a sufficient amount of toner to pass through the aperture to form an image dot. Thus, the known apparatus suffers from a low image forming speed. In this respect, an increase in the magnitude of the electric field within the aperture in an effort to promote the passage of the toner particles through the aperture would require the voltage applying means to employ expensive drive elements for applying a sufficiently high potential to the control electrodes of the particle flow modulator.